Everything about Shingi's character design choices is laced with melancholy.
Let's start with a simple observation:
1. The character for "shin"(神) - divine – is phonetically similar to the "Shi"(死) – death.
As a matter of fact, the word for "dead" in Japanese is pronounced "shin" (死ん). There's the missing "n"! On can argue that Shingi, this divine ritual, can also be considered a death-related ritual since it involves the slaying of a mononoke. There's always a sadness involved in its killing, as a mononoke is birthed from intense suffering. I'm pretty sure that this phonetic similarity is not a coincidence, given how much care is put into naming the other characters in Mononoke Karakasa (see my analysis on Utayama and Mizorogi)
Now let's dive into more observations below the cut!